An implantable blood pump used as a mechanical circulatory support device or “MCSD” includes a pumping mechanism to move blood. The pumping mechanism may be a radial flow pump, such as the HVAD® Pump manufactured by HeartWare Inc. in Miami Lakes, Fla., USA. The HVAD® Pump is further discussed in U.S. Pat. No. 8,512,013, the disclosure of which is hereby incorporated herein in its entirety. Alternatively, the pumping mechanism may be an axial flow pump, such as the MVAD® Pump, also manufactured by HeartWare Inc., and the pumps described in U.S. Pat. Nos. 7,972,122, 8,007,254 and 8,419,609, the disclosures of which are also hereby incorporated herein in their entirety, or any other pump suitable for providing vascular assistance. In operation, the blood pump draws blood from a source such as the right ventricle, left ventricle, right atrium, or left atrium of a patient's heart and propels the blood into an artery such as the patient's ascending aorta or peripheral artery. Due to the nature of the application, the pumping mechanism must be highly reliable. Patient comfort is also a significant consideration. In addition to the pumping mechanism, the device may include a controller and the drive electronics for the pumping mechanism. The controller and drive electronics may receive power from an external power source. That power may be used to drive a motor of the pumping mechanism at a desired speed.
In some cases, the blood pump may provide only partial support to the patient. In such cases, the patient's heart continues to pump blood from the left ventricle to the aorta through the aortic valve (or, in the case of the right ventricle, to the pulmonary artery through the pulmonic valve), and the blood pump further assists the activity of the patient's heart in parallel. Although the heart only pumps blood into the aorta during systole, the blood pump works during both systole and diastole, when the aortic valve is open or closed. Thus, over a given period of time including systole and diastole, the patient's heart and the blood pump may each be responsible for some of the work performed to pump blood to the patient's arteries.
It is generally understood that increasing the speed of the blood pump causes the pump to perform more work. In some cases, increasing the speed of the pump may be beneficial for the patient, by allowing the pump to perform additional work in tandem with the heart. However, in other cases, the pump may already be operating at a speed at which there is little or no additional work to be performed (e.g., blood to be pumped from the ventricle), in which case increasing patient's heart may simply cause the patient's heart to perform less work, but not necessarily increase the overall work performed. In some cases, increasing motor speed of the blood pump may leave so little work for the heart that the aortic valve is not forced open during systole, thereby transitioning the pump from a state of partial-assistance to a state of full-assistance. Such changes in work performed by the heart may be unwanted. Therefore, it is desirable to determine how much work is being performed by each of the heart and the blood pump, so that it may further be determined whether it is desirable to increase a motor speed of the pump.
Conventionally, the amount of work performed by the each of the patient's heart and the blood pump may be determined based on invasive measurements. For example, ventricular work could be assessed using catheter-based measurements. The catheter-based measurements could be used to construct a pressure-volume loop (“PV loop”) indicating the total work performed by the patient's heart. FIG. 1 is a diagram of an example PV loop, showing the volume of blood stored in the patient's left ventricle (“LVV” horizontal axis, measured in microliters) and the pressure exerted by the left ventricle (“LVP” vertical axis, measured in mmHg) over the course of a single cardiac cycle. In the example of FIG. 1, stroke work (“SW”) exemplifies the amount of work performed by the heart. The potential energy (“PE”) may be considered an indication of the amount of work not performed by the heart, and therefore remaining for the pump to perform.
The above example demonstrates catheter-based measurements for the “left” half of the heart. Similar measurements may be taken for the “right” half of the heart (e.g., right ventricular volume, right ventricular pressure, pulmonary pressure, etc.).
Invasive measurements may also be used to detect a situation in which the blood pump transitions from a state of partial-assistance to a state of full-assistance. For example, catheter-based measurements of left ventricular pressure (“LVP”) and aortic pressure (“AOP”) may be used to identify aortic valve closure during systole. If LVP and AOP measurements cross over one another during the course of the patient's cardiac cycle (particularly during the transitions between systole and diastole), the cross over is an indication that the aortic valve has opened, thereby causing AOP and LVP to be relatively the same (as compared to during diastole). By contrast, if LVP and AOP do not cross over, that is an indication that the aortic valve has not opened even during systole, since LVP remains below AOP. However, it is impractical to monitor the patient's heart and pump function invasively after the blood pump has already been implanted, particularly while the patient is outside of a clinic or hospital.